In the chemical industry it is necessary, in many instances, that relatively pure compounds be employed as starting materials in chemical reactions. It is therefore necessary to separate desirable chemical compounds from a mixture of other chemical compounds in which the aforesaid desired compounds are present. As an example of this, in some refining operations aromatic compounds are derived from certain processes. The main synthesis routes for the processes of these aromatics usually comprise reformed naphthas and hydrogenated pyrolysis gasolines. However, the aromatic compounds such as benzene, toluene, and the isomeric xylenes are present in admixture with other nonaromatic hydrocarbons such as paraffins or cycloparaffins. In order to recover the desired aromatic compounds in relatively pure form, it was necessary to effect a fractional distillation of the mixture. However, the separation of aromatics from the conventional sources usually could not be effected by a conventional distillation inasmuch as the nonaromatic hydrocarbons present in the feeds possess boiling points which may be within the same boiling range as that of the aromatics. In addition, many of the nonaromatic hydrocarbons present in the mixture have a tendency to form azeotropes with the aromatic compounds, thus contributing to the difficulty of separation. When utilizing distillation procedures for the separation, losses of the aromatic compounds were relatively high and the product purity was low. Therefore, the demand for these aromatic compounds as intermediates in the production of other materials was limited by the low purity of the compounds as well as limited availability.
As was hereinbefore set forth, high grade or high purity aromatic compounds are utilized as intermediates in the preparation of many compounds which are used in the commercial world today. Pure aromatic compounds such as benzene, toluene and the xylenes comprise primarily feedstocks in the petrochemical industry, particularly the plastic and synthetic fiber industry. For example, benzene may be used as the intermediate in the preparation of styrene which itself finds use in the production of many plastics. Likewise, benzene is used as an intermediate in the preparation of phenol, synthetic detergents, Nylon intermediates, aniline, insecticides, fumigants, as well as in motor fuels for upgrading the octane number thereof. Toluene also finds use as a high octane blending stock for gasolines, as a solvent for paints and coatings, gums, resins, in medicines, dyes, perfumes, saccharine, explosives, etc.; while xylenes may be used also as a blending agent for gasoline such as aviation gasoline with individual or specific xylenes such as paraxylene being used in the synthesis of terephthalic acid, for the production of synthetic resins and fibers such as Dacron, Mylar, Terylene, etc.
In order to obtain the aromatic compounds in a desirable state of purity, a liquid/liquid extraction for the separation of aromatics as a class from other hydrocarbons was developed. Various solvents have been employed in this extraction process. A major requirement for the selection of a solvent for the recovery of aromatics from mixtures of hydrocarbons containing these desired compounds involves the relationship of solubility between aromatic compounds and nonaromatic compounds such as paraffins and cycloparaffins. Various solvents have been employed to effect this liquid/liquid separation, however, the characteristics of these solvents with respect to aromatic selectivity/solubility are widely varied. When effecting the extraction process, it is possible to alter the solubility characteristics of the solvent by controlling the operating parameters under which the extraction is effected. However, if the solubility characteristic is allowed to become too high, the selectivity becomes too low for the required separation to be effected cleanly and efficiently, thus resulting in excessive processing requirements and equipment which is in a position downstream of the extractor. The practical effects which may result when utilizing a solvent which possesses the most desirable characteristics will include a lower solvent circulation rate than is required when utilizing other solvents to give the same separation efficiency, thus resulting in a lowering of utility requirements and a reduction in investment costs inasmuch as the equipment throughput in the solvent circulation section is decreased as compared to other solvents. Another practical consideration which is to be considered is the initial cost of the solvent itself.
Heretofore, one solvent which has been employed in a highly efficient manner to effect a liquid/liquid extraction of aromatic compounds from hydrocarbon mixtures is sulfolane which is tetrahydrothiophene-1,1-dioxide. However, the cost of sulfolane is relatively high when compared to other solvents which may be obtained by utilizing starting materials which are less costly, thus resulting in an overall lower cost of the desired solvent material.
It has now been discovered that other solvents comprising aliphatic sulfoxides which possess a specific configuration of the type hereinafter set forth in greater detail may be utilized as solvent extractants for the separation of aromatic compounds from nonaromatic compounds. The fact that these particular sulfoxides possess the desirable characteristics which enable them to be employed as solvent extractants was unexpected in view of the inability of other aliphatic sulfoxides to act as solvent extractants, the differences in solvent extractability and selectivity being hereinafter set forth in greater detail. The particular solvents of the present invention which comprise dialkyldisulfinyl-, dialkyldisulfonyl- or alkylsulfinylalkylsulfonylalkanes may be prepared from starting materials which are relatively inexpensive, the alkanes possessing desirable characteristics such as a relatively high specific gravity, a low specific heat, a low melting point as well as a boiling point which is higher than that of the heaviest aromatic compounds which are to be extracted from mixtures of hydrocarbons.